1. Field of the Invention
This invention relates to the reactive sputtering of a layer of a metal compound on a substrate. More particularly, this invention relates to process and apparatus for reactive sputtering a layer of a stoichiometric metal compound on a semiconductor wafer including control of the voltage used to sputter the metal from a target, as well as expediting the flow of the reactive gas to the sputtered metal atoms to form the layer of stoichiometric metal compound.
2. Description of the Related Art
The sputtering of a metal target, such as, for example, a titanium target, using a power source such as, for example, a planar magnetron to provide a source of constant power, in a reactive gas atmosphere such as nitrogen (N.sub.2), is a well known technique for high rate deposition of a film of a metal compound such as, for example, titanium nitride (TiN). However, control of such a reactive sputtering process, to both maximize the rate of deposition or film formation, as well as to control the film stoichiometry, has been difficult to achieve.
Reactive sputtering is a very versatile coating technique that allows the preparation of a wide variety of compound materials such as, for example, TiN, using a titanium target and N.sub.2 as the reactive gas. However, it has traditionally had one major drawback. When the partial pressure of the reactive gas, such as N.sub.2, reaches the right level to form a stoichiometric film of the metal compound on the surface of a substrate, it also forms the same metal compound on the surface of the metal target. This, in turn, causes the partial pressure of the reactive gas in the chamber to rise, since the formation of the metal compound on the surface of the target results in less of the target metal being available to react with the reactive gas in the chamber. This is due both to the sputter rate of the metal target being changed by the presence of the metal compound on the target surface, resulting in less metal ions being sputtered off the target and therefore available to react with the reactive gas, as well as a reduction of metal target surface available to react with the reactive gas.
Such changes in the stoichiometry of the constituents of the film of metal compound being formed, based on both the rise in reactive gas concentration, as well as the change in sputter rate of the target due to the formation of the metal compound thereon which, in turn changes the concentration of sputtered metal ions available to react with the reactive gas, result in an undesirable formation of film on the substrate which, while containing both sputtered metal atoms and reactive gas atoms, does not possess the proper stoichiometry to form a homogeneous layer of the desired stoichiometric metal compound.
It has previously been proposed to control the reactive sputtering of titanium to form a TiN film by monitoring the partial pressure of the N.sub.2 in the chamber and to decrease the N.sub.2 flow upon sensing an increase in nitrogen pressure. This is described more fully by H. F. Hmiel in "Partial Pressure Control of Reactively Sputtered Titanium Nitride", published in the Journal of Vacuum Science Technology, A 3(3), May/June 1985, at pages 592-595. The author describes the use of a quadrupole mass spectrometer together with microprocessor-controlled argon and nitrogen flow valves to control the flow of both argon and nitrogen into the sputtering chamber to control the sputtering process.
Unfortunately, the response time of this type of system is rather slow since the sampled gas from the sputtering chamber must be analyzed in the mass spectrometer for nitrogen content and the result then fed into the microprocessor which, in turn, must then adjust the flow of nitrogen into the sputtering chamber to eventually effect the desired change in nitrogen concentration in the chamber.
Prior to this, the control of reactive sputtering of AlN films using an aluminum target and an Ar/N.sub.2 sputtering gas mixture was studied by McMahon, Affinito, and Parsons in "Voltage Controlled, Reactive Planar Magnetron Sputtering of AlN Thin Films", published in the Journal of Vacuum Science Technology, A 20(3), March 1982, at pages 376-378; and by Affinito and Parsons in "Mechanisms of Voltage Controlled, Reactive Planar Magnetron Sputtering of Al in Ar/N.sub.2 and Ar/O.sub.2 Atmospheres", published in the Journal of Vacuum Science Technology, A 2(3), July-Sept. 1984, at pages 1275-1284.
In those two articles, control of gas feed, power, current, and voltage to achieve formation of stoichiometric thin films of AlN by reactive sputtering of aluminum in an argon/nitrogen mixture is discussed. In the earlier article, the authors concluded that constant gas flow rates and control of the voltage was best suited for reactive planar magnetron sputtering of aluminum.
In the latter article (by two of the authors of the earlier article), the authors stated that they had previously found that a single valued, monotonic functional relationship exists between V (cathode voltage) and P (reactive gas partial pressure) for the Al-Ar/N.sub.2 system, while a range exists where each value of I (current) corresponds to three values of V or P and that when the discharge was operated by controlling V, they were able to operate over the full range of I-V-P combinations. However, the authors then noted that subsequent experiments with Al, Zn, In, and Mo on Ar/O.sub.2 atmospheres revealed that voltage control was not possible across the transition between bare metal and completely covered target states. They also noted that in Ar/N.sub.2 atmospheres, control was not possible with Mo but wa possible with Zn.
There remains a need for improvement in the control of the process for the formation and deposition of a a layer of a stoichiometric metal compound on a semiconductor wafer by reactive sputtering.